The purpose of this study is to investigate, at a detailed clinical level, the under-utilization of coronary angiography in the Los Angeles County Medical System. The system provides health care to a population,of predominantly Black, Hispanic, and poor white residents of Los Angeles County. While several studies focusing on clinical appropriateness have documented excess-utilization of medical and surgical procedures, no studies have addressed the issue of under-utilization at a clinical level. A set of criteria for establishing the clinical necessity of coronary angiography, developed at the RAND Corporation, will be used to study, at a detailed clinical level, the presence and level of under-- utilization of coronary angiography in the Los Angeles County Medical System. Medical records of patients with positive stress tests with indications for which angiography is necessary will be abstracted to determine the percentage of these patients who underwent the procedure at each facility. Patients will be contacted by mail and asked to verify whether they under-went coronary angiography, and to provide information about other cardiac-related procedures and events they May have experienced. Factors such as age, ethnicity, payment source, health status and patient preferences known to influence the utilization of medical procedures will be examined. The results of this study will provide for the first time clinically detailed evidence of the possible under-utilization of coronary angiography among Blacks, Hispanics and other poor individuals.